Question

Hmmm. Consider this program, whose goal is to figure out the best way to get the bottom 16 bits of an integer, as a signed integer.

public class SignExtend16 {
    public static int get16Bits(int x)
    { 
        return (x & 0xffff) - ((x & 0x8000) << 1);
    }

    public static int get16Bits0(int x)
    { 
        return (int)(short)(x);
    }    

    public static void main(String[] args)
    {
        for (String s : args)
        {
            int x = Integer.parseInt(s);
            System.out.printf("%08x => %08x, %08x\n",
               x, get16Bits0(x), get16Bits(x));
        }
    }
}

I was going to use the code in get16Bits0, but I got the warning “Unnecessary cast from short to int” with Eclipse’s compiler and it makes me suspicious, thinking the compiler could optimize out my “unnecessary cast”. When I run it on my machine using Eclipse’s compiler, I get identical results from both functions that behave the way I intended (test arguments: 1 1111 11111 33333 66666 99999 ). Does this mean I can use get16Bits0? (with suitable warnings to future code maintainers) I’ve always assumed that JRE and compiler behavior are both machine-independent for arithmetic, but this case is testing my faith somewhat.

Answer 1

Since numeric casts are implicit-friendly, I think the only reason you’re getting the warning is that the compiler will always make the cast to int upon return, making your explicit cast redundant.